Jitter is the deviation from an ideal timing edge of the actual timing edge in a sequence of data bits that typically occurs at high frequencies. Jitter in a digital system is essentially a timing error that can affect the timing allocation within a bit cell. Jitter is typically measured at the differential zero crossings for balanced electrical signals, at the average voltage level for unbalanced signals, and at the average optical power level for optical signals. Jitter is often used as a figure of merit, and tracking jitter-induced errors over a period of time can provide an indication of system stability.
There are various types of jitter, such as random jitter, periodic jitter, and data-dependent jitter (“DDJ”). DDJ produces different amounts of jitter for different digital outputs. For example, a digital output of “00010001” would have a different amount of DDJ than a digital output of “10101010” from the same digital source because the latter digital output has more transitions, and hence contains more high-frequency components in its spectrum. The digital patterns with higher frequency content will be attenuated and phase shifted relative to the lower frequency patterns. Determining the level(s) and type(s) of jitter are important in characterizing components used in digital systems. In general, digital systems having higher transmission rates (typically expressed in Mb/s or Gb/s) have timing margins that are less tolerant to jitter.
In telecommunications (e.g. SONET, SDH. or OTN) and enterprise (e.g. Ethernet) applications, jitter specifications and measurements are documented through standards bodies. In the high-speed I/O arena, many new bus standards are being introduced with little commonality in specifying and measuring jitter. Similarly, characterization of high-speed serial electrical backplanes is gaining increased attention as their use increases for high-bandwidth interconnections. Jitter is often the limiting factor for electrical backplanes operating in the 1-10 Gb/s range. Measuring the jitter on a data stream is desirable in order to characterize digital designs.